Various methods for generating separating fissures in substrate surfaces are known from the art. Laid Open Application DE 10 2008 000 418 A1 discloses a method in which the surface of a structural component part is heated locally along a predetermined fracture line by means of radiation of a laser source or infrared radiation source. In this case, the radiation energy is applied asymmetrically, the predetermined fracture line being acted upon at short time intervals by two radiation energies of identical or different strengths. In conjunction with a shock cooling carried out subsequently, the formation of a separating fissure takes place along the predetermined fracture line.
A method for cutting brittle bodies described in DE 43 05 107 A1 uses laser radiation to heat the surface of the body along a cutting line. The thermomechanical stress which is built up in this way leads to fracture of the body along the cutting line. The cross section of the laser beam acting on the surface has an elongated shape and is oriented along the cutting line. The length and width of the beam cross section is shaped by optics, and the length can be additionally delimited by variable diaphragms.
In the laser method shown in WO 96/20062 A1 and the apparatus suitable for this method, a guided superficial separating fissure is produced in the material by generating a locally limited thermoelastic stress, and the substrate can subsequently be broken along this separating fissure. The thermal stresses occur between a zone of the substrate surface which is heated locally by suitable laser radiation and in which compressive stresses are generated and a subsequent shock cooling which is realized by a directed coolant jet and by means of which tensile stresses are generated. The controlled separating fissure is formed when the region with the greatest stress difference moves relative to the substrate surface proceeding from an artificially produced initiating defect. A substrate which is broken along this separating fissure has a high-quality break edge which is completely free from edge damage compared to conventional scribing-and-breaking methods using small carbide wheels or diamonds.
The apparatus disclosed in the above-cited WO96/20062 A1 comprises a suitable laser beam source whose radiation is partially absorbed by the substrate so that a local heating can take place. The laser radiation is directed to the substrate surface. The laser beam is shaped by beam-shaping optics to form an elliptical beam spot on the substrate surface, and the length of the major axis of the ellipse oriented in direction of the relative movement is a multiple of the length of the minor axis oriented transverse to the relative movement. To generate the stress difference, the point of impingement of the directed coolant jet follows the elliptical beam spot in direction of the relative movement. The relative movement between the optics with the downstream coolant jet and the substrate is likewise realized by the apparatus. To this end, the substrate is moved linearly relative to the optics. The specific adjustment of process parameters such as movement speed, laser power, beam spot shaping and cooling parameters allows the formation of the separating fissure to be influenced in a deliberate manner.
In Laid Open Application DE 10 2007 033 242 A1, the basic principle of the method described above is used to cut a flat substrate into a plurality of rectangular sections of optional size. This is done by generating preferably orthogonally intersecting separating fissures which are generated in a first machining direction and second machining direction. The separating fissures are always started at an initiating artificially produced defect at an outer edge of the substrate and end again also at an outer edge of the substrate. The machining is carried out on the upper side of preferably small substrates which are placed on a horizontally oriented supporting surface and which have the usual dimensions of semiconductor wafers. The relative movement between the optics and the substrate is carried out by means of a movement of the supporting surface, and therefore also of the substrate supported thereon, on a linear axis of the apparatus. To adjust the second machining direction, the substrate is rotated around a perpendicular axis of rotation of the apparatus. The relative movement for generating the separating fissure in the second machining direction takes place subsequently with the same linear axis as for the first machining direction. For larger substrates, for example, flat glass substrates which have the standardized dimensions for float glass production and which are to be provided with separating fissures in two machining directions, the relative movement and the rotation of the substrate relative to the optics could only be carried out by moving the supporting surface or the substrate itself at a high cost of construction and with a correspondingly large space requirement.
The above-mentioned problem relating to rotational movement can be solved by the apparatus disclosed in Laid Open Application DE 10 2005 027 800 A1. In this case, in order to realize the second machining direction which is preferably oriented orthogonal to the first machining direction, the optics are integrated in a machining head with which the beam spot and the coolant jet aligned with the beam spot can be rotated synchronously around a perpendicular axis of rotation. In order to bring about the relative movement in both machining directions, it is necessary to provide a second linear axis which is oriented orthogonal to the first linear axis and by means of which the substrate can be moved.
The use of two directed coolant jets in the above-cited DE 10 2005 027 800 A1 is to be regarded as a further advantage over the apparatus from DE 10 2007 033 242 A1. For this purpose, a coolant jet is arranged at both ends of the major axis of the elliptical beam spot symmetric to the beam axis. This arrangement allows the substrate to be machined in alternating senses in one machining direction by always using only the respective coolant jet following the beam spot in the machining direction. Accordingly, separating fissures can be generated from opposing substrate edges in an alternating manner. In this apparatus also, the relative movement is carried out by moving the substrate or supporting surface. Thus it follows that only substrates having small dimensions can be machined in this case also.
Use of the apparatus requires that a separating fissure always starts at an outer edge of the substrate and also ends at an outer edge of the substrate starting from an initiating defect. For this reason, the relative movement is carried out in such a way that the corresponding linear axis is always moved from a predetermined starting point to a predetermined target point. To achieve the thermoelastic stress at the starting and end points of the separating fissures that is needed for generating the separating fissures, these points must be covered by the whole length of the elliptical beam spot extending in the major axis and by the coolant jet. Therefore, the starting point and target point of the linear axes must be predetermined in such a way that the entire beam spot always lies outside the substrate at the starting point and end point of the separating fissure. In terms of control, the apparatus is not designed to have a separating fissure start or end, for example, in the middle of the substrate surface.
Moreover, continuous separating fissures bring about the result that the only cuts that can be made between two separating fissures which are adjacent in a machining direction are those whose edges which are oriented in the first machining direction and second machining direction have the same length as the cuts which are adjacent in the corresponding direction. Therefore, it is not possible for cuts differing from the edge lengths of adjacent cuts in at least one machining direction to be made from a substrate.
For certain technical applications it would be advantageous if separating fissures could start and end at any points on the substrate surface. Thus, for instance, the substrate surfaces could be exploited in an optimal manner according to need by cuts arranged in a nested manner. With a nested arrangement, separating fissures in one machining direction could have starting points and end points at separating fissures in the other machining direction so that a T-shaped intersection is formed. As has already been noted, using the apparatuses and methods identified from the prior art requires that the whole length of the elliptical beam spot extending in the major axis and the coolant jet must run over the starting point and end point of a separating fissure. Therefore, in case of very long elliptical beam spots, the distance to be run over is also very long. As a result, the problem arises for T-shaped intersections that the beam spot impinges on the separating fissure extending transverse to the machining direction and on the adjoining portion of the substrate that is not to be severed. This can cause breaking defects in the subsequent breaking process, and the cut extending transverse to the machining direction may be damaged.